Technical Field
The present disclosure relates to an imaging apparatus and a method for controlling the imaging apparatus. The present disclosure also relates to a technique for obtaining a pupil division image using an image sensor configured of a microlens corresponding to a plurality of photoelectric conversion units, and more particularly to a technique for preferably correcting distortion of an image caused by saturation occurring in each of the photoelectric conversion units of a pixel.
Description of the Related Art
Conventionally, there has been provided a technique for performing focus detection using an image sensor configured of a microlens corresponding to a plurality of photoelectric conversion units, in which pupil-divided images are obtained and a phase difference between the two obtained pupil-divided images is acquired.
For example, Japanese Patent Application Laid-Open No. 2001-83407 discusses a technique for generating a view image by a conventional signal processing technique, in which focus detection is performed by acquiring a phase difference of pupil-divided images while all values of photoelectric conversion units corresponding to the same microlens are added up to treat that addition value as a value of one pixel so that the pixel array thereof is the same as that of the conventional imaging pixel.
In Japanese Patent No. 4691930, after a value corresponding to the electric charge generated at a part of photoelectric conversion units within the same microlens is read in a non-saturated state, an addition value corresponding to the electric charges generated at all of the photoelectric conversion units within the same microlens is read. Then, a value of other photoelectric conversion units is estimated from a difference value between the read values, so that a phase-difference image signal can be obtained while the high-sensitivity characteristics of an imaging pixel signal are maintained.
However, in the above-described conventional technique discussed in Japanese Patent No. 4691930, there arises a problem in which a signal waveform is distorted remarkably due to saturation that occurs when gain is increased in order to have higher International Standards Organization (ISO) sensitivity.
For example, in a case where an image sensor designed for ISO 100 is used in ISO 200, a signal amplitude thereof is doubled by increasing gain.
In ISO200, with half the saturation level of electric charges that can be stored in a pixel of the image sensor, an analog-digital (AD) converter exceeds its conversion range to reach the apparent saturation level. An amount of electric charges that can be stored in the division pixel as the addition value thereof is up to twice as much as the amount of AD conversion range. In such a state, even if a value of one division pixel is subtracted from an addition value of the division pixels, a value of another division pixel cannot be restored.
In an extreme case, if both the addition value of electric charges generated at all photoelectric conversion units and the value of the electric charges of one photoelectric conversion unit reach the saturation level, this gives rise to a phenomenon in which another value thereof becomes zero when the one value is subtracted from the addition value. In such a case, even if the focus detection is performed by acquiring the phase difference of two pupil-divided images obtained in such a state, the focus detection cannot be performed correctly.